WASHINGTON : Fighters are ditching al-Qaeda affiliates in Yemen and Africa to join the Islamic State that has seized territory in Iraq and Syria and are now being targeted in American airstrikes, US officials said.The officials view the movements as a worrisome indication of the expanding appeal of the Islamic State of Iraq and Syria that has overwhelmed military forces in the region and may now see itself in direct conflict with the US, The Washington Post said in a dispatch published Sunday."Small groups from a number of al Qaeda affiliates have defected to ISIS," as the group is also known, the newspaper said, citing a US official with access to classified intelligence assessments. "And this problem will probably become more acute as ISIS continues to rack up victories." The Post said the influx has strengthened the Islamic State, also known as Islamic State of Iraq and Syria or ISIS. Citing US officials, the newspaper said the defections to the Islamic State have "come primarily from al Qaeda in the Arabian Peninsula, the Yemen-based group that has launched several bombing plots targeting the US, and al-Qaeda in the Islamic Maghreb, which had seized territory in northern Mali before facing strikes carried out by France last year." "It's not to the point where it's causing splintering within the affiliates," said the senior US counterterrorism official. But the defections have accelerated in recent months, officials said, and also involve fighters from groups in Libya and elsewhere that are not formally part of al Qaeda.Last month, Pakistan-based Tehreek-e-Khilafat became the first jihadi outfit in the South Asia to break ranks from al Qaeda and declare its allegiance to the Islamic State. Al Qaeda has distanced itself from the group, chiding it for its lack of teamwork in its aggressive, brutal expansion. "US officials attribute the Islamic State's rapid emergence to factors both psychological and tactical... And its merciless reputation triggered rampant defections among Sunni members of Iraq's security forces already disenchanted with the government in Baghdad," the Post said.Counterterrorism analysts at the CIA and other agencies have so far seen no indication that an entire al Qaeda node or any of its senior leaders are prepared to switch sides. But officials said they have begun watching for signs of such a development, the paper added.US officials estimate that the Islamic State has as many as 10,000 fighters, including 3,000 to 5,000 from countries beyond its base in Iraq and Syria. Its ranks have swelled with the emergence of the civil war in Syria - a country relatively easy to reach from both in the Middle East and Europe - as a larger magnet for militants than Afghanistan or Iraq were. The group has also attracted critical support from disenfranchised Sunni residents in Mosul and other Iraqi cities, civilians who have lost patience with the government of Iraqi President Nouri al-Maliki but may not embrace the hard-line agenda of the Islamic State.